The present invention relates generally to semiconductor fabrication and more specifically to formation of buried straps.
As semiconductor device length shrinks, a shallow junction is desired to satisfy circuit operation. Buried strap out-diffusion serves as a path for the connect trench active area (AA). The current method to form the buried strap out-diffusion is to perform an angle ion implantation into silicon into the buried strap polysilicon (poly) and then to do a drive-in anneal.
U.S. Pat. No. 6,200,873 B1 to Schrems et al. describes a production method for a trench capacitor with an insulation collar and a buried strap process.
U.S. Pat. No. 6,156,606 to Michaelis describes a buried strap process and uses gas phase doping (GPD) for a capacitor doping step.
U.S. Pat. No. 6,018,174 to Schrems et al. describes a buried strap process that uses GPD to dope trench sidewalls.
U.S. Pat. No. 6,211,020 B1 to Tripsas et al. describes a plasma doping process that is used to dope high aspect sidewalls.
U.S. Pat. No. 5,945,704 to Schrems et al. describes a trench capacitor with an epitaxial silicon (EPI) buried layer.
Accordingly, it is an object of one or more embodiments of the present invention to provide an improved method of forming buried strap out-diffusion areas.
Other objects will appear hereinafter.
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, a substrate having a pad oxide layer formed thereover is provided. A masking layer is formed over the pad oxide layer. The masking layer, pad oxide layer and substrate are etched to form a trench within the substrate. The trench having an outer sidewall and an upper portion. The upper portion of the trench is lined with a collar. A poly plate is formed within the trench. The poly plate and collar are etched below the substrate to form a recessed poly plate and a recessed collar and exposing a portion of outer sidewall of trench. Ions are implanted into the substrate through exposed outer sidewall of trench by gas phase doping. An SiN sidewall layer is formed over the exposed outer sidewall of trench at a temperature sufficient to diffuse the implanted ions further into the substrate to form the buried strap.